A known evaporative emission control system for a fuel system of an internal combustion engine that powers an automotive vehicle comprises an evaporative emission containment space for containing volatile fuel vapors and a purge valve through which the fuel vapors are purged from the evaporative emission containment space to an intake system of the engine for combustion. The evaporative emission containment space includes headspace of a fuel tank that contains a supply of volatile liquid fuel for the engine and an associated fuel vapor collection canister, e.g. a charcoal canister, through which the tank headspace is vented to atmosphere.
The purge valve opens when conditions are conducive to purging, commnicating the evaporative emission containment space to the engine intake system. Atmospheric venting of the tank headspace maintains the tank headspace pressure near atmospheric. Intake system vacuum communicated through the open purge valve draws gases present in the evaporative emission containment space (a mixture of fuel vapors and air) through the purge valve and into the intake system. There the purge flow entrains with intake flow into the engine, ultimately to be disposed of by combustion within the engine. A known purge valve comprises an electric actuator that receives a control signal developed by an engine management computer to open the purge valve in the proper amount for various operating conditions, thereby developing the desired purge flow.
Because the evaporative emission control system relies solely on intake system vacuum to draw fuel vapors from the evaporative emission containment space, the intensity of the vacuum directly effects the purge flow rate. At larger vacuum intensities, the engine management computer can adjust the purge valve to compensate for changes in vacuum. However, when system vacuum falls below a certain threshold that is determined by various factors, there is insufficient pressure differential between the evaporative emission containment space and the intake system to develop the requisite purge flow.
Some automotive vehicle internal combustion engines may develop nominal intake system vacuums that range from about 10 inches Hg to about 20 inches Hg. Purge valves used with such engines are designed for such a range. For any one or more of various reasons however, actual intake system vacuum in a particular engine may be incapable of exhibiting that nominal range. That characteristic may impair operation of an evaporative emission control system because there is insufficient pressure differential to develop the desired purge flows. An engine that has direct high-pressure gasoline fuel injection may exhibit a nominal system vacuum range that is much closer to atmospheric pressure than the nominal range of intake system vacuum for other engines.